1. Field of the Invention
The present invention relates, generally, to a check valve for a metal injection molding machine or die casting machine, and more particularly, but not exclusively, the invention relates to a seal for such a valve and particularly a lip seal for such a valve.
2. Background Information
The state of the art includes many check valves for both plastic and metal injection molding machines. While many of these check valves work satisfactorily in the plastics environment, most do not work well in the metal injection environment. Plastic is quite viscous and does not tend to flow through small gaps. Molten metals are much hotter and have a much lower viscosity so that any tolerable gap must be much smaller than an acceptable gap for plastic molding. This requires much tighter tolerances for metal molding. Up to the present time, no satisfactory seal for a check valve for metal molding has been developed. The check valves in current use do not satisfactorily meet the dual requirements of very low bypass leakage and the ability to withstand operating within an environment that imparts extremely harsh mechanical loading, chemical reactivity and high temperature. The check valves currently used have a very short operational life and must be replaced often leading to substantial disruption of the molding or casting process and reduction in the production of satisfactory parts.
The following patent references are representative of sealing alternatives currently available.
U.S. Pat. No. 5,865,442 issued Feb. 2, 1999 to Iwashita describes a piston seal formed upon one side of a piston body. The piston body has a tapered surface receiving a back portion of the seal. The front portion has a lip. The seal is compressed against the wall of the cylinder by the combined forces applied by the tapered surface and oil pressure on the lip portion. The seal is vulcanized adhered to the piston.
U.S. Pat. No. 2,742,333 issued Apr. 17,1956 to Taylor et al describes a plastic seal that is molded into a groove on a piston. The seal has a lip portion that is forced into tight sealing contact with a bore of the cylinder by action of an O-ring positioned on the piston.
U.S. Pat. No. 4,231,578 issued Nov. 4, 1980 to Traub describes a sealing assembly for sealing a shaft. The seal comprises a first sealing ring having a Y-shaped cross-sectional configuration and a second sealing ring having a generally L-shaped cross-sectional configuration. The two rings interface along the L-shaped portion. The. Y-shaped seal is made of rubber and the L-shaped ring of polytetrafluorethylene.
U.S. Pat. No. 4,743,033 issued May 10, 1988 describes a seal assembly for subterranean wells. A first non-elastomeric sealing element has a hub portion secured to the piston and an outwardly flaring skirt portion searingly engage able with the cylinder bore. A secondary sealing element of resilient metal is secured to the piston and defines a frusto-conical lip portion that snugly engages the skirt portion of the first element.
U.S. Pat. No. 5,507,505 issued Apr. 16, 1996 to Stein et al describes a lip seal that has a series of concentric grooves formed in the surface of the lip contacting the wall of a polymeric body.
While each of these references teaches the use of lip seals in a particular environment, none of them would be capable of withstanding the heat, pressure and corrosiveness involved in molding metal parts. It is even doubtful that any of them could operate satisfactorily in a plastic injection molding machine environment.
There is a need for a check valve in metal injection molding and die casting that is durable and can withstand the high temperatures, injection pressures and corrosive environment and effectively seals the injection channel to prevent the backflow of the molten metal into the supply cylinder during the injection stroke. None of the above referenced patents describe a check valve seal nor a sealing device that could be modified to effectively seal such a check valve during injection of metal into a mold.